The following apparatuses have been known as the fuel supply control system of an internal combustion engine.
More specifically, an intake air flow quantity Q or an intake air pressure PB is detected as the state quantity participating in sucked air, and based on this detected value and the detection value of the engine revolution number N, the basic fuel supply quantity Tp is computed. This basic fuel supply quantity is corrected based on various coefficients sets COEF based on the driving states, such as the engine temperature represented by the cooling water temperature, the air-fuel ratio feedback correction coefficient LMD set based on the air-fuel ratio in the sucked air-fuel mixture detected through the oxygen concentration in the exhaust gas and a correction portion Ts for correcting the opening-closing delay of the fuel injection valve caused by changes of the battery voltage, and the final fuel supply quantity Ti is thus computer (Ti.rarw.Tp.times.COEF.times.LMD+Ts) and this computed quantity of a fuel is intermittently supplied to the engine by the fuel injection valve (see, for example, Japanese Unexamined Patent Publication No. 60-240840).
The air-fuel ratio feedback correction coefficient is set, for example, by the proportional-integral control, and in the case where the actual air-fuel ratio detected through the oxygen concentration in the exhaust gases by an oxygen sensor is richer (leaner) than the target air-fuel ratio (theoretical air-fuel ratio), the air-fuel ratio feedback correction coefficient LMD is first decreased (increased) only by the predetermined proportional portion P and is then decreased (increased) by the predetermined integral portion I synchronously with the time or synchronously with the revolution of the engine, and the control is performed so that the actual air-fuel ratio is reversed repeatedly in the vicinity of the target air-fuel ratio.
In an electromagnetic fuel injection valve ordinarily used for injecting and supplying a fuel into an engine, the flow quantity characteristics are changed with the lapse of time or by intrusion of foreign substances or clogging of injection holes, and even in the state of new products, there is present a dispersion of about .+-.6% in the flow characteristics because of a production tolerance.
Accordingly, in the case where injection valves are disposed independently for respective cylinders, even if the driving control is carried out in all the cylinders based on the same fuel supply quantity, because of the above-mentioned dispersion of the flow quantity characteristics, there is caused a dispersion of the quantity of the practically injected and supplied fuel among the respective cylinders.
However, according to the conventional air-fuel ratio feedback control, an oxygen sensor is arranged at the junction of exhaust gas paths of the respective cylinders, the average air-fuel ratio in the respective cylinders is detected based on the oxygen concentration in exhaust gases detected by the oxygen sensor and the control is made to bring this average air-fuel ratio close to the target air-fuel ratio. Accordingly, the dispersion of the flow quantity characteristics among the fuel injection valves of the respective cylinders cannot be corrected, and if there is a dispersion of the flow quantity characteristics, it is impossible to obtain the target air-fuel ratio in the respective cylinders.
More specifically, for example, if the flow quantity of one cylinder is reduced because of clogging of injection holes and the average air-fuel ratio becomes lean, in order to compensate this reduction of the average air-fuel ratio, the fuel supply quantity is uniformly increased in all of the cylinders and the air-fuel ratio in other normal cylinders becomes rich. Accordingly, if there is a dispersion of the flow quantity characteristics in the respective cylinders, the average air-fuel ratio can be feedback-controlled to the target value, but it is impossible to realize the target air-fuel ratio in the respective cylinders. Therefore, if there is brought about a dispersion of the air-fuel ratio in the respective cylinders, the property and state of exhaust gas are worsened, the stability of the engine driving is degraded, and there is a risk of a misfire in a specific cylinder.
The present invention has been completed to solve the above-mentioned problem, and it is an object of the present invention to provide an error-detecting apparatus for detecting a dispersion (error) of fuel supply characteristics in respective cylinders in a fuel supply control system equipped with a function of performing the feedback control of the air-fuel ratio, a learning apparatus for correcting the fuel injection quantity for respective cylinders based on the result of this detection and controlling the air-fuel ratios in the respective cylinders separately to the target air-fuel ratio, and a diagnosis apparatus for diagnosing fuel supply means of the respective cylinders separately on receipt of the detection and learning results.